There is known a chair of this type provided with a backrest and comprising an inner shell consisting of a seat portion and a backrest portion formed into an integral body, with an outer shell consisting of a plurality of sections fixed to the under surface and the back surface of the inner shell. In particular, in the conventional chair the outer shell is divided into at least a portion for a backrest to cover the back surface of the inner shell and a portion for a seat to cover the under surface of the seat portion of the inner shell. Particularly, in many of those chairs in which the motion of the seat relative to the leg is complicated, the outer shell for the seat is divided into a plurality of pieces, with a wide gap formed between the pieces. In some of the chairs the outer shell for the seat is fixed to both the inner shell and a supporting unit for supporting the inner shell.
However, with an outer shell divided into a seat portion and a backrest portion, it is difficult to provide the outer shell with a sufficient strength for the whole shell. In particular, the conventional outer shell has a function merely as a cover for covering the back and under surfaces of the inner shell, and the inner shell is so designed as to provide a structural strength of the seat and the backrest. The conventional inner shell is provided with various complex ribs for suppressing deformation and various cutouts for helping deformation, thereby to impart a required strength to each component member. However, there is a limit to imparting different degrees of strength to different parts of the inner shell which is an integral body. This poses a problem that there is little degree of freedom in design.
The primary object of the invention is to solve the above-mentioned problem by forming the seat portion and the backrest portion of the outer shell into an integral body.
Another object of the invention is to solve concrete problems posed by an outer shell formed as an integral body. The problems are as follows:
1 Outer shells of different shapes are required for accommodating supporting units of different types to be contained between the inner and outer shells, so that a plurality of large dies of different types need be prepared for forming outer shells.
2 If the outer shell is applied to a chair in which the seat shifts relative to a supporting unit, a wide gap is likely to remain between the outer shell and that portion of the supporting unit which comes out of the shell.
3 Since the outer shell itself functions as a member which bears a load, it is difficult to provide the backrest itself with a suitable resiliency. In particular, in conventional chairs, the outer shell for a backrest is fixed to the rear surface of the inner shell by engagement of pins with holes each having a claw for preventing the pin from falling off or by means of screws. If such an arrangement is used in the backrest portion of the outer shell of the invention, the flexibility of the backrest is likely to be lost.
4 With a chair provided with a backrest with little flexibility and without any particular measures, when the backrest is inclined rearwardly, the distance between the reference point of the sitting position which is the center of the load on the seat and the backrest point corresponding to the fifth lumbar vertebra of a person on the seat varies. In particular, in the chair which takes a reference position, even if the backrest point is set to an ideal position, when the backrest is inclined rearwardly by the user with his back against the backrest, the backrest point shifts upwardly relative to the back of the user, so that the user feels uncomfortable. Moreover, when the user repeatedly leans his back against the backrest and raises his back therefrom, the shirt, etc. worn by the user may be lifted up and the lower portion of the shirt is pulled out from his trousers.
5 To allow proper resilient rearward deformation of the backrest, with both the inner and outer shells being L-shaped in side view, it is desirable that the backrest portion of the outer shell slightly shifts downwardly relative to the backrest portion of the inner shell upon rearward inclination of the backrest. With this arrangement, however, when the backrest is inclined rearwardly, the seat portion of the outer shell is apt to move forwardly relative to the seat portion of the inner shell. Therefore, if bolts are fixed to the outer shell perpendicularly to the outer surface thereof, a bending force acts on the bolts. Such a bending force repeatedly acts on the bolts each time the backrest is inclined, so that a problem arises that the tapped holes are widened to cause the bolts to become loose.
6 If the inner shell is combined with the outer shell so that they may be displaced relative to each other, such displacement acts somewhere on the shells, so that the outer shell disadvantageously expands outwardly thereby to form a gap between the outer shell and the inner shell or the cushion material. For example, if the backrest is made flexible, with the backrest portion of the outer shell being so arranged as to be shiftable downwardly within a predetermined distance relative to the backrest portion of the inner shell, when the upper portion of the backrest is inclined rearwardly relative to the lower portion thereof, a bordering portion between the backrest portion and the seat portion of the outer shell protrudes rearwardly, so that a gap is likely to be formed in this portion.
7 In the conventional inner shell at least four corners of the seat portion thereof are fixed to the metallic bracket of a supporting unit, so that the whole structure lacks flexibility. If an outer shell L-shaped in side view is fixed to the inner shell of the above-mentioned construction, its flexibility is further reduced with resulting reduction of the cushioning ability.
8 In a chair having a seat and a backrest formed into an integral body by connecting an inner shell L-shaped in side view to an outer shell L-shaped in side view, it is difficult to embed a bearing or the like for supporting armrests in the side portions of the seat and the backrest, so that it is difficult to use inverted L-shaped armrests having one end supported on the seat and the other end supported on the backrest.
9 In conventional chairs, usually a manual controller is mounted on a particular member of the outer shell, and a mechanism to be controlled by the manual controller is provided in a supporting unit. If such an arrangement is adopted in an outer shell of an integral type L-shaped in side view, the manual controller must be detached from the outer shell when the outer shell is removed from the inner shell, so that it not only takes much time to mount the outer shell on or remove it from the inner shell, but also it is impossible to confirm the function of the mechanism by operating the manual controller mounted in place on the outer shell while looking into the mechanism.
10 That portion of the supporting unit which supports the inner shell and a backrest support projecting from the supporting unit must be enclosed in a relatively small space formed between the inner shell and the outer shell. As a result, a problem arises that it is rather difficult to make the backrest support and the supporting unit rigid enough to form a strong structure free from loosening.